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Syphilis Rates Not Result of Unsafe Sex

British researchers says natural cycle of disease might explain recent U.S. spike

WEDNESDAY, Jan. 26, 2005 (HealthDayNews) -- The epidemic of syphilis among American gay men may be a product of the natural evolution of the disease, not the result of condom fatigue and more unsafe sex, British researchers suggest.

According to their study, published in the Jan. 26 issue of Nature, statistics from 68 American cities show that syphilis rates go up and down in regular cycles. The researchers suspect the disease loses steam as previously infected people develop temporary immunity, but then recovers a few years later when there are more vulnerable people in the population.

The findings should help public health officials get a better handle on how to understand syphilis outbreaks, said study co-author Nicholas Grassly, a researcher at Imperial College London. "They need to be interpreted carefully, since they do not always reflect changes in behavior or the environment."

The number of syphilis cases in the United States reached almost 7,100 in 2003, the third annual increase in a row. While syphilis is easily cured with drugs, officials have raised an alarm because the disease appears to make it easier to transmit the virus that causes AIDS.

Grassly said researchers were inspired to do the study by an unusual phenomenon: gonorrhea rates haven't gone up like syphilis rates. This struck the researchers as odd, especially considering that studies have suggested American gay men -- vulnerable to both diseases -- have been abandoning safer sex practices as drug treatments make AIDS less of a fatal disease.

The researchers analyzed both syphilis and gonorrhea statistics from 68 U.S. cities and focused on the years 1960 to 1993, because they encompassed the sexual revolution and the gay liberation movement, both of which have been suspected as possible contributors to syphilis epidemics.

In many cities, syphilis rates followed a regular pattern, peaking every 10 years or so. Gonorrhea, however, didn't follow any regular pattern.

Why not? The researchers think that's because some people develop resistance to syphilis infection, at least for a while, after becoming ill. At some point, syphilis begins to run out of people to infect. (Some public health Web sites say there's no such thing as syphilis immunity, but the researchers pointed to a 2003 study that suggests it does exist.)

There's no immunity to gonorrhea, however, and people can get it repeatedly.

The "troughs" suggest that health officials can try to eliminate syphilis during its down cycles, Grassly said. Indeed, rates dipped so low during the 1990s that the U.S. Centers for Disease Control and Prevention hoped to virtually kill off the disease by the middle of this decade. However, rates quickly grew among gay and bisexual men over the past several years, throwing a wrench into the government's plans.

Grassly said his findings don't have any direct implications for the AIDS epidemic. Unlike those with syphilis, people infected with HIV can't be cured and don't develop immunity.

However, he added, "someone infected with syphilis is more susceptible to HIV and more likely to transmit HIV to their sexual partner."

Dr. Michael Horberg, director of HIV/AIDS policy for the Kaiser Permanente Health Plan, said the findings don't change the fact that the best way to avoid syphilis is to take precautions, like using condoms.

"There is still an epidemic within the gay community," Horberg said. "Even if these numbers are following natural trends of immunity, transmission will still go down with appropriate emphasis on safer sex."

More information

Learn more about syphilis from the U.S. Centers for Disease Control and Prevention.

SOURCES: Nicholas Grassly, M.Sc., Imperial College London, England; Michael Horberg, M.D., director, HIV/AIDS policy, Kaiser Permanente Health Plan, Santa Clara, Calif.; Jan. 26, 2005, Nature
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